This invention is related to a seat assembly for a valve having an expanding gate assembly and employing a floating type seat assembly. More specifically, this invention is related to expanding gate valves wherein the gate assembly has a gate member and an attached segment member and outer sides of the gate assembly are substantially parallel and expand outwardly in a parallel relation in both the open position and the closed position.
In the prior art the seats for this expanding type gate valve are generally pressed into the valve body in recesses around the flow passageways at the valve chamber so that theoretically the seat will be in a fixed position. Ideally the seats provide essentially parallel surfaces for the gate to contact when it is in the expanded configuration regardless of whether the gate is in the open position or the closed position. As a practical matter, however, the seats are not always, fixed and they can float or move inward toward the gate assembly under high differential pressure conditions. In doing this the seats move to a position tight against the gate assembly and drag excessively on it thus making the valve very difficult to open or close. The other construction used with this type of valve is a floating seat arrangement wherein the seat member is designed to float in the seat pocket generally in the direction of the flow passageway so that it can be moved against the gate sides by the fluid pressure. The disadvantage of this type design is that excessive drag is always created between the seat members and the gate assembly when the gate assembly is moved due to the upstream seat being forced against the gate assembly by fluid pressure at all times. This is a particularly serious problem in high pressure service applications because large forces are needed on the gate to move it in order to overcome the drag forces of the seats. The advantage in the floating seat design is that the sealing surfaces of the seats will easily align with the sealing surfaces of the gate because of clearances that provide for the floating action of the seats, therefore, less wedging pressure in required of the expanding gate assembly to seal the valve. Heretofore, no practical seat construction has been known for parallel sided expanding gate valves which will take advantage of the easy sealing characteristics of the floating seat design yet not have the excessive drag disadvantage which is inherent of the design.